1. Field of the Invention
The present invention relates to a dielectric filter with variable central frequency comprising at least one dielectric resonator formed from a dielectric ceramic body covered with electrodes as well as signal input and output electromagnetic coupling means to and from the filter.
2. Description of the Prior Art
Ultra high frequency (UHF) filters are widely used today, for example in telecommunications for choosing a given frequency band width. In frequencies up to about 700 MHz, filters comprising piezoelectric resonators may be used. However, for the whole of the UHF band width, extending up to 3 GHz such filters are no longer appropriate and dielectric filters are then used which have similar performances to cavity filters but which are much less costly and space consuming. In addition, for some applications the pass band of piezoelectric filters is too narrow.
Dielectric filters of this type have been described in the French application No. 82 18236 filed on Oct. 23rd 1982 in the name of THOMSON-CSF. Such parallelepipedic type filters use compositions of the type SnTiO.sub.4.
Using such parallelepipedic resonators, band pass filters may be formed by disposing a plurality of resonators side by side. Generally, it is preferable to use resonators of the quarter wave type because of their shorter length. The electrode metallizations of these resonators are respectively parallel to each other and perpendicular to the insulating substrate on which said substrates are fixed. Coupling between these resonators is provided by mutual inductance. The connection to the input and to the output of the filter may be provided either by soldering respectively the first and the last resonator directly to the electrode, or by means of a loop placed above the first and last resonators as an exciter and a collector respectively. In in order to obtain a filter having a given pass band, resonators are used having a natural and given frequency and said resonators are coupled depending on the coupling coefficients required by calculation of the filter, by carefully adjusting the distance between the resonators. When it is desired to form a multichannel filter using such resonators, a plurality of resonators are generally placed close to one another. The natural frequency of each resonator is then tuned to the different frequencies of the channels and it is sufficient to switch the input and the output of the filter to the desired resonator for selecting the corresponding channel. Such a solution however requires the use of as many resonators as there are channels, and per filter pole. This results in a filter of considerable size which becomes prohibitive when the number of channels desired is greater than two.